I realized my interest in bone biology during my Ph.D. and Post Doctoral years. Since joining my Mentor's lab, I have gained considerable knowledge in stem cell biology. Thus, I have decided to combine my background in bone biology with my current studies of stem cells to investigate the hematopoietic stem cells (HSCs) as a potential therapy for bone disorders. Long-term, the proposed studies will help me to distinguish myself as an independent investigator and develop my own area of research. Even though I am proficient in execution of techniques, I am still lacking in planning and critical assessment of experiments and in preparation of manuscripts. Further training under my Mentor and Co-Mentor will solidify my knowledge in stem cell biology, and help me to achieve a greater understanding and mastery of experimental design and grantsmanship, a critical step before I can realize more independence as an academic scientist. In addition to my Mentor and Co-Mentor, I have established collaborations with other senior investigators with expertise specifically relevant to the proposed studies. I also have full Departmental support for this application including career development programs and use of core facilities. Support from the Mentored Research Scientist Development Award will allow me to focus on this continued training over the next few critical years to further both my own career development and the investigation of the aims of this proposal. I have a strong desire to develop a successful career and carve a niche for myself in the area of basic and translational bone research. The receipt of the NIH support at this stage of my training is critical to my realization of this goal. This application will focus on Osteogensis Imperfecta (OI) and the role of HSCs as a potential therapy for this disease. OI or brittle bone disease, a genetic disorder resulting from the abnormal amount and/or structure of Type I collagen, is the most common hereditary bone disease. It is characterized by moderately to extremely fragile bones which may fracture upon little or no trauma, skeletal deformities, osteopenia and short stature. Presently there is no cure for OI. Ideally, therapy for OI should be directed towards improving bone strength by improving the production and integrity of the secreted collagen. Preclinical studies have demonstrated that bone marrow contains cells that can engraft and become competent osteoblasts after transplantation. Clinical trials using bone marrow stem cells in a small number of children with severe forms of OI have shown promising results with increased growth velocity, total body mineral content and fewer fractures. Although bone marrow transplantation shows potential, there is a controversy regarding the cell involved. Current dogma states that bone marrow contains two types of stem cells, HSCs and mesenchymal stem cells (MSCs), and that their repertoire of reconstituting potentials are distinct and separate from each other; i.e. HSCs produce blood cells and few other cells like mast cells and osteoclasts, while MSCs generate a number of mesenchymal cells including fibroblasts, adipocytes, chondrocytes and osteoblasts. Recently, studies have begun to question the distinction between the potentials of HSCs and MSCs in generating osteoblasts and osteocytes. My Mentor has developed a novel transplantation model in which the bone marrow of lethally irradiated recipient mice is reconstituted by a clonal population of cells derived from a single EGFP+ HSC. Studies using this model have demonstrated that many types of tissue fibroblasts/ myofibroblasts as well as adipocytes are derived from HSCs, suggesting an HSC contribution to multiple mesenchymal tissues. Preliminary in vivo data also supports an HSC origin for osteoblasts and osteocytes. Therefore, we hypothesize that the primary defect in OI is in the HSC. We propose to test this hypothesis using single HSC transplantation in combination with a mouse model of OI, the oim/oim mouse. Based on our hypothesis, HSC transplantation should lead to replacement of affected osteoblasts with normal cells, in turn correcting collagen defects in bone and ameliorating and/or preventing OI pathologies. Long term, these findings have the potential to identify HSC transplantation, and not the transplantation of MSCs or progenitor cells, as the therapy of choice for this collagen disorder.